Differential expression of histone sequences in Drosophila following heat shock

The expression of the sequences encoding the four nucleosomal histone proteins was examined following heat shock of a variety of Drosophila cells and was found to be highly differential. In Drosophila melanogaster KC-O cells grown in suspension culture, there is a continuation of the synthesis of all four of the nucleosomal histone proteins following heat shock. Analysis of RNA from these cells confirms that histone messengers are transcribed and located on polysomes. This exact same pattern of histone protein synthesis occurs in KC-O cells grown to low density on plates. In contrast, KC-O cells grown to high density on plates exhibit a dramatic elevation of H2b protein synthesis relative to the synthesis of the other core histones. Organs from D melanogaster third instar larvae were examined to ascertain whether histone protein synthesis continues following heat shock in the organism. Different tissue types exhibited differential histone synthesis. Imaginal disks excised from heat-shocked larvae continue to synthesize nucleosomal histones in a variable fashion. In contrast, neither fat bodies, brains, nor salivary glands continues to synthesize core histone proteins at a significant level. D hydei plated cell cultures and larval tissues fail to synthesize histones at any detectable level following a heat shock. Based on these observations, we propose that there is a differential synthesis of nucleosomal proteins in Drosophila that is highly dependent on the state of the cells prior to the heat shock.